So I have a website where we offer computer services and have since 2009, you can confirm this by checking whois on our domain. We mostly accept payment by paypal or googlecheckout, although if neither can be used we will accept other payment options.

So one day we get an email from a prospective customer where he says we are probably a scam and will scam him blah blah blah, we reply that no we will not and he can check whois and google to see message board threads going back years to establish we have been around a while. Also give him the usual info on our offerings and prices and times etc.

Then he emails back that he is interested BUT will only pay via Western Union so that he will be able to cancel payment as paypal is not secure This is basically totally wrong, you have a lot more recourse with paypal but whatever.
We tell him ok even though this guy is seeming fishy already, then he emails back say he will pay with WU AFTER we do the work and he is satisfied with it and we tell him sorry not going to happen. Now he calls our phone number and leaves <<78>> voice mails most dead air in the course of a couple hours along with several more emails with insults and at this point I just spam folder this guy and forget him obviously a flake.

That was two months ago, he still sends a email every few days saying fuck you and go fuck yourself and other insults. WTF? I'm tempted to tell him to get a fucking life, and that googling some porn and fucking himself would be a far better use of time than harassing us forever.

Now he calls our phone number and leaves <<78>> voice mails most dead air in the course of a couple hours along with several more emails with insults and at this point I just spam folder this guy and forget him obviously a flake.

That was two months ago, he still sends a email every few days saying fuck you and go fuck yourself and other insults. WTF? I'm tempted to tell him to get a fucking life, and that googling some porn and fucking himself would be a far better use of time than harassing us forever.

What a nut!

Read Gavin de Becker's book The Gift of Fear, where he talks a lot about this kind of nutcase. If you respond to this guy, all you will have done is taught him that if he want to get a response from you, he should send you 78 voice mails and an email every few days and you will answer him. You'll never ever be rid of him, you'll just make him more persistent. Just keep ignoring him and eventually he will move on to someone who responds.

Or configure Outlook to automatically move messages from his email address to a folder called "nutcase". I think GMail will do the same thing.

I think all of the web-based mail services will do this. It's also a very good idea because if his crazy emails escalate to something worse, you've got documentation that this has been an ongoing problem. Even if it never comes to that, you still have something in case it does.